I would group all your items temporarily (to make sure they stay positioned relative to each other), then modify the document settings to be square, then center the grouped items on the square page vertically and horizontally.
Actually, I don't really like this suggestion as it only works if all
your content is on one layer.
Offtopic: is there a reason to work with layers? Is there a memory savings on that? grouping has always been sufficient for me...